I mean they could have at the very beginning at the least and then changed it when he found infinity. The multiplayer Mark VI armor is far more similar to the Halo 3 model than what was forced upon us in the campaign.
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In all seriousness though I believe that SMAC array got destroyed in the Battle For Earth. They're very expensive seeing as only 2 planets had them and I think Reach Reach only had 20 but Earth was supposed to have 300. And with most of the Navy budget going to Infinity I Guess they haven't had the time or funds to rebuild them.